Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 141

by 10 mOsm or more; ingestion of salt likewise increases

plasma osmolality, whereas plasma osmolality is lowered by

drinking water.

When a person becomes dehydrated, the blood becomes

more concentrated and the total blood volume is reduced. The

increased plasma osmolality and osmotic pressure stimulate

osmoreceptors, which are neurons mainly located in a part

of the brain called the hypothalamus (chapter 8, section 8.3).

During dehydration, water leaves the osmoreceptor neurons

because of the increased osmolality of the extracellular fluid.

This causes the osmoreceptors to shrink, which mechanically

stimulates them to increase their production of nerve impulses.

As a result of increased osmoreceptor stimulation, the per-

son becomes thirsty and, if water is available, drinks. Along

with increased water intake, a person who is dehydrated

excretes a lower volume of urine. This occurs as a result of the

following sequence of events ( fig. 6.14 ):

1. Increased plasma osmolality stimulates osmoreceptors in

the hypothalamus of the brain.

2. The osmoreceptors in the hypothalamus then stimulate

a tract of axons that terminate in the posterior pituitary;

Figure 6.13 Red blood cells in isotonic, hypotonic,
and hypertonic solutions. In each case, the external solution
has an equal, lower, or higher osmotic pressure, respectively,
than the intracellular fluid. As a result, water moves by osmosis
into the red blood cells placed in hypotonic solutions, causing
them to swell and even to burst. Similarly, water moves out of
red blood cells placed in a hypertonic solution, causing them to
shrink and become crenated.

Isotonic solution

Hypotonic solution Hypertonic solution

H 2 O

H 2 O

CLINICAL APPLICATION

Intravenous fluids are generally isotonic to blood plasma

in order to prevent cells from expanding or shrinking from

water movement. Common intravenous fluids are normal

saline and 5% dextrose, which, as previously described,

have about the same osmolality as normal plasma (approx-

imately 300 mOsm). Another isotonic solution frequently

used in hospitals is Ringer’s lactate. This contains lactate

as well as Na^1 ,  Cl^2 ,  K^1 , and Ca^2 1 ions to more closely

resemble the electrolytes in plasma. In contrast to these

solutions, mannitol (derived from the sugar mannose) is

sometimes given intravenously because it is osmotically

active, helping to draw water from swollen tissues in the

head when there is cerebral edema due to trauma or stroke,

among its other uses.

Regulation of Blood Osmolality

A relatively constant osmolality of extracellular fluid must be
maintained. This is mostly because neurons could be damaged
by swelling or shrinkage of the brain within the skull, and
because neural activity is altered by changes in the concentra-
tions of ions (chapter 7). A variety of mechanisms defend the
homeostasis of plasma osmolality, usually preventing it from
changing by more than 1% to 3%. For example, dehydration
due to strenuous exercise can increase plasma osmolality

Figure 6.14 Homeostasis of plasma

concentration. An increase in plasma osmolality (increased

concentration and osmotic pressure) due to dehydration

stimulates thirst and increased ADH secretion. These effects

cause the person to drink more and urinate less. The blood

volume, as a result, is increased while the plasma osmolality is

decreased. These effects help to bring the blood volume back

to the normal range and complete the negative feedback loop

(indicated by a negative sign).

Dehydration

Blood volume –

Plasma osmolality

Osmoreceptors in the

hypothalamus

ADH secretion

from posterior

pituitary

Thirst

Kidneys Drinking

Water intake

Water retention

Sensor

Integrating center

Effector